Re: [RFC][PATCH] sched: Fix race in task_group()

Hillf Danton
2012-06-27T12:40:05Z

The patch went three versions, the first,

On Fri, Jun 22, 2012 at 7:36 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> Reported-by: Stefan Bader <stefan.bader@canonical.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 32157b9..77437d4 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1246,6 +1246,9 @@ struct task_struct {
> const struct sched_class *sched_class;
> struct sched_entity se;
> struct sched_rt_entity rt;
> +#ifdef CONFIG_CGROUP_SCHED
> + struct task_struct *sched_task_group;
> +#endif
>

The second,

>> On 26.06.2012 15:48, Peter Zijlstra wrote:
>> Here's one that's actually compile tested (with the right CONFIG_foo
>> enabled) and I fixed the autogroup lockdep splat.
>>
>> ---
>> Subject: sched: Fix race in task_group()
>> From: Peter Zijlstra <peterz@infradead.org>
>> Date: Fri, 22 Jun 2012 13:36:05 +0200
>>
>> Reported-by: Stefan Bader <stefan.bader@canonical.com>
>> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
>> ---
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -1246,6 +1246,9 @@ struct task_struct {
>>       const struct sched_class *sched_class;
>>       struct sched_entity se;
>>       struct sched_rt_entity rt;
>> +#ifdef CONFIG_CGROUP_SCHED
>> +     struct task_group *sched_task_group;
>> +#endif
>>

And the third, https://lkml.org/lkml/2012/6/26/331

>From d751ab1f1e532f32412d99b71a1bfea3e5282d07 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 22 Jun 2012 13:36:00 +0200
Subject: [PATCH] sched: Fix race in task_group()

Stefan reported a crash on a kernel before a3e5d1091c1 ("sched: Don't
call task_group() too many times in set_task_rq()"), he found the reason
to be that the multiple task_group() invocations in set_task_rq()
returned different values.

Looking at all that I found a lack of serialization and plain wrong
comments.

The below tries to fix it using an extra pointer which is updated under
the appropriate scheduler locks. Its not pretty, but I can't really see
another way given how all the cgroup stuff works.

Reported-and-tested-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
[backported to apply to 3.0 and 3.2]
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
---
include/linux/init_task.h | 12 +++++++++++-
include/linux/sched.h | 5 ++++-
kernel/sched.c | 32 ++++++++++++++++++--------------
3 files changed, 33 insertions(+), 16 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 56de5c1..1fd9884 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1242,6 +1242,9 @@ struct task_struct {
const struct sched_class *sched_class;
struct sched_entity se;
struct sched_rt_entity rt;
+#ifdef CONFIG_CGROUP_SCHED
+ struct task_struct *sched_task_group;
+#endif

where sched_task_group was defined to be task_struct twice(in the first
and the third versions) and to be task_group once.

Before backport, feel free to respin with the final define determined.